228 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



normal head, a second head, which had an oral disc 

 and tentacles but no mouth (Fig. 12, p. 52). The ten- 

 tacles never learned that no mouth was present, but 

 continued when meat was offered to make the attempt 

 to force it into an opening that did not exist. 



Some reactions of lower animals cannot be repeated 

 indefinitely. We must not conclude, however, that 

 this is due to processes of association and that the 

 animal has learned certain effects. It is a well-known 

 fact that many worms that live in cases suddenly 

 withdraw into their cases when a shadow is cast on 

 them. I analysed this process and showed that the 

 shadow has nothing to do with the phenomenon. It 

 is due to a reaction against negative variations in the 

 intensity of the light, comparable to the " break-con- 

 traction " of a muscle. The experiment does not suc- 

 ceed if repeated an indefinite number of times in 

 succession. Nagel concludes from this that these 

 worms possess " the ability to judge." " The animal 

 recognises that the shadow cast so frequently does 

 not signify the approach of an enemy or of any other 

 danger" (Nagel). In reality these reactions are in- 

 herited forms of irritability that have nothing to do 

 with experience. The reason that the reaction ceases 

 if repeated frequently is due to a simple after-effect 

 of the stimulus, a case that we often meet in the physi- 

 ology of both animals and plants. The assumption 

 that such low animals as eyeless worms and snails 

 possess ideas or even the one idea of " an approach- 

 ing enemy or other impending danger " is entirely 



